For example, in an electric vehicle (EV) traveling using an electric motor, an electric vehicle such as a hybrid vehicle (HEV) that travels with the combined use of the electric motor and an engine as a power source, secondary batteries such as lithium ion rechargeable batteries and nickel hydride rechargeable batteries are installed.
Such a secondary battery is deteriorated by repeating charging and discharging, and the storageable capacity (current capacity, electric power capacity, etc.) gradually decreases. Then, in the electric vehicle, etc. using secondary battery, by detecting the degree of deterioration of the secondary battery, strageable capacity is obtained, calculating the travelable distance with the secondary battery, the lifetime of the secondary battery, and the like.
As one indicator of the degree of deterioration of the secondary battery used is SOH (State of Health) which is a ratio of a current strageable capacity to an initial strageable capacity. It is known that this SOH is correlated with an internal resistance of the secondary battery. Therefore, detecting the internal resistance of the secondary battery enables to obtain the SOH based on this internal resistance.
Generally, the internal resistance is very small, so it was difficult to obtain sufficient detection accuracy, and Patent Document 1 discloses a battery state detection device in which the detection accuracy of the internal resistance is enhanced.
FIG. 7 is a diagram showing a schematic configuration of a battery state detection device 500 described in Patent Document 1. The secondary battery B to be detected has an electromotive force portion e generating a voltage and an internal resistance r. Detecting this internal resistance r can obtain the SOH of the secondary battery B.
The secondary battery B generates a voltage V between both electrodes (positive electrode Bp and negative electrode Bn), which the voltage V is determined by a voltage Ve generated by the electromotive force generated by the electromotive force portion e and a voltage Vr generated by a current flow through the internal resistance r (V=Ve+Vr). The negative electrode Bn of the secondary battery B is connected to the reference potential G.
The battery state detection device 500 includes a differential amplification unit 511, a changeover switch 512, a first capacitor 513, a second capacitor 514, a charging unit 515, a first analog-digital convertor (ADC) 521, a second analog-to-digital converter (ADC) 522, and a microcomputer (μCOM) 540.
In the configuration shown in this figure, when the μCOM 540 transmits a charge start control signal through the output port PO2 to the charging unit 515, the charging unit 515 starts to flow a predetermined constant charging current Ic. As a result, charging of the secondary battery B is started.
When charging is started, the μCOM 540 controls the changeover switch 512 so that the positive electrode Bp of the secondary battery B and the first capacitor 513 are connected. As a result, the first capacitor 513 is charged and held (sampled and held) with the voltage V1=Ve+r·Ic between the two electrodes of the secondary battery B during charging.
Next, when the voltage both the electrodes of the secondary battery B acquired through the input port PI1 reaches the predetermined state detection voltage, the μCOM 540 controls the changeover switch 512 through the output port PO1, and connects the positive electrode Bp of the secondary battery B and the second capacitor 514, as well as transmits a charge stop control signal to the charging unit 515 through the output port PO2.
As a result, when the charging current Ic to the secondary battery B is stopped and the storage state of the second capacitor 514 is stabilized, the second capacitor 514 is sampled and held with the voltage V2=Ve between the two electrodes of the secondary battery B during stop of charging.
In this state, the μCOM 540 detects a differential voltage Vm the differential amplifier 511 outputs through the input port PI2. Then, the detected difference voltage Vm is divided by an amplification factor A of the differential amplifier 511, further divided by the charging current Ic, and the internal resistance r of the secondary battery B (=(Vm/Av)/Ic) is detected.
Finally, the μCOM 540 transmits a control signal of charging start signal to the charging unit 515 through the output port PO2. The charging unit 515 starts to flow the predetermined constant charging current Ic again in the secondary battery B according to this control signal. As a result, charging is started and the battery state detection process finishes.